city of cape town 35c/2016/17: cape flats wwtw access …
TRANSCRIPT
Prepared by:
JG AFRIKA (PTY) LTD
Cape Town Branch
PINELANDS
7430
Tel: (021) 530 1800
Email: [email protected]
Project leader: Chris Wise
CITY OF CAPE TOWN
35C/2016/17: CAPE FLATS WWTW ACCESS ROAD
PROJECT
STORMWATER MANAGEMENT REVIEW
29 MARCH 2021
VERIFICATION PAGE Form 4.3.1
Rev 1
TITLE: 35C/2016/17: CAPE FLATS WWTW ACCESS ROAD
STORMWATER MANAGEMENT REVIEW
JGA REF. NO. DATE: REPORT STATUS
4932 29/03/2021 Revision 1
CARRIED OUT BY: COMMISSIONED BY:
JG AFRIKA (PTY) LTD
PO Box 38561
PINELANDS
7430
CITY OF CAPE TOWN: WASTEWATER BRANCH
4th Floor, Water City
Cnr Mike Pienaar and Voortrekker Roads
Bellville
Tel: (021) 530 1800
Fax: (021) 532 0950
Email: [email protected]
Tel: (+27) 83 728 3180
AUTHOR(S) CLIENT CONTACT PERSON
N. Goncalves M. Marias
SYNOPSIS
Stormwater review for proposed access road to the Cape Flats Waste Water Treatment Works
(WWTW), Cape Town.
KEY WORDS:
Stormwater, hydrology,
© COPYRIGHT: JG Afrika (Pty) Ltd.
QUALITY VERIFICATION
This report has been prepared under the controls established by a quality management
system that meets the requirements of ISO9001: 2008 which has been independently
certified by DEKRA Certification under certificate number 90906882
Verification Capacity Name Signature Date
By Author Technologist N. Goncalves
Checked by: Technical Director D. Petersen
Authorised by: Technical Director C.C. Wise
Filename: X:\Projects - (In-progress)\4932 Cape Flats WWTW Access (CW)\05 Docs&Reports\Stormwater Management Review
Page i
CITY OF CAPE TOWN
CAPE FLATS WWTW ACCESS ROAD
STORMWATER MANAGEMENT REVIEW
TABLE OF CONTENTS
1 PRE-DEVELOPMENT CONDITIONS ................................................................................1
1.1 Existing Conditions .................................................................................................. 1
1.2 Drainage and Infiltration ......................................................................................... 1
1.3 Stormwater Quality ................................................................................................. 2
2 APPLICABLE STANDARDS AND DESIGN DATA ...............................................................2
3 PROPOSED DEVELOPMENT ..........................................................................................2
3.1 Land Use .................................................................................................................. 2
3.2 Planned infrastructure ............................................................................................ 2
3.3 Management of stormwater impacts ..................................................................... 3
4 CONCLUSIONS & RECOMMENDATIONS .......................................................................4
5 REFERENCES ................................................................................................................4
TABLES
Table 1-1: Design Depth-Duration-Frequency Values ................................................................ 1
FIGURES
Figure 3-1: Access road routing in relation to trapped low-points ............................................ 3
CITY OF CAPE TOWN
CAPE FLATS WWTW ACCESS ROAD
STORMWATER MANAGEMENT REVIEW
1 PRE-DEVELOPMENT CONDITIONS
1.1 Existing Conditions
The site is located in an open, currently undeveloped area, just northeast of the existing inlet works
structure. The potential catchment area is characterised by rolling dune topography, numerous
naturally occurring trapped low-points and no clear drainage path across the site. Due to this
topography, the broader catchment area — which is defined by Strandfontein Rd. to the east,
Pelican Park to the north, Zeekoevlei and Cape Flats WWTW to the west, and a rolling dune line to
the south — flow to several low-points within the site creating pockets of isolated sub-catchments.
1.2 Drainage and Infiltration
High permeability and infiltration is typical of this area. Consisting of unconsolidated sandy deposits
comprising sands, with calcretes, calcarenite, marine gravel, clay and silcrete horizons at varying
depths in a succession of the order of 25m to 35m thick in this area.
The site is located close to the naturally occurring drainage line associated with the Zeekoevlei.
Regionally groundwater for the area is encountered within two potential aquifers within the Cape
Flats area: (1) An upper, intergranular unconfined aquifer, situated within the unconsolidated
transported sandy soils. Depth of this intergranular aquifer tends to vary seasonally, and (2) A
deeper-lying fractured rock aquifer of the Malmesbury Group. Groundwater quality tends to vary
throughout the Cape Flats area, but is generally moderate.
The City of Cape Town’s Depth-Duration-Frequency rainfall grid (2010) (which factors in climate
change) was used to determine the applicable design storm depth for various return periods. The
quantities are tabulated in Table 1-1.
Storm
duration (min)
Rainfall depth (mm) per duration per return period
Return period (years)
2 5 10 20 50 100
5 5.6 7.6 9.0 10.5 12.5 14.1
10 8.1 10.8 12.8 14.8 17.8 20.1
15 9.9 13.3 15.8 18.3 22.0 24.8
30 13.7 18.3 21.6 25.2 30.1 34.2
45 16.4 22.1 26.1 30.4 36.3 41.2
60 18.7 25.2 29.8 34.6 41.5 47.0
1440 (24-hr) 62.68 84.06 99.59 115.80 138.58 157.20
Table 1-1: Design Depth-Duration-Frequency Values
With no clear drainage path and no large catchment areas, peak flows are expected to be relatively
minor and divided between sub-catchments around the site. Proposed road section A to B
encounters trapped low-points that require balancing, section B to C runs along a ridgeline splitting
overland run-off, section C to D encounters trapped low-points that require balancing, section D to
E follows existing access tracks encountering little to no trapped low-points or overland run-off.
1.3 Stormwater Quality
The pre-development runoff can be considered average quality, given the location of the site in an
undeveloped area. At this stage:
• No site specific geotechnical investigation has been conducted to establish the presence of
illegally dumped builder’s rubble and domestic solid waste that would have had a negative
impact on the quality of the subsurface water. Although, a reasonable expectation can
anticipate the presence of some waste material.
• No analysis of the stormwater has been conducted to determine its quality.
The extent of this impact (as well as the determination of appropriate mitigation measures) are
outside the scope of this stormwater review, but may warrant further investigation by the City of
Cape Town
2 APPLICABLE STANDARDS AND DESIGN DATA
The management of stormwater for the proposed development will be done in accordance with the
following standards and design data:
1. Minimum Standards for Civil Engineering Services in Townships Version 1, July 2013, City of
Cape Town – Transport for Cape Town;
2. Guidelines for Human Settlement Planning and Design Volume 2, 2000, CSIR – Department
of Housing;
3. Management of Urban Stormwater Impacts Policy Version 1.1, May 2009, City of Cape Town:
Roads & Stormwater;
4. Stormwater Management Planning and Design Guidelines for New Developments, 2002, City
of Cape Town: Transport Roads & Stormwater;
3 PROPOSED DEVELOPMENT
3.1 Land Use
The proposed development intends to include a 6.8m wide road within a 10m road reserve, to
provide access road to the Cape Flats WWTW.
3.2 Planned infrastructure
The proposed access road has been designed with a cambered profile and a finished road level
elevated above the surrounding natural ground level. Runoff from minor events will drain to either
side of the road and will be conveyed by open side drains to low-points where the road does go
through cut. Over the short stretch where the proposed access road is routed through a trapped
low-point, culverts will be sized and constructed to allow for the adequate balancing of stormwater
run-off as well as for environmental purposes.
Figure 3-1: Access road routing in relation to trapped low-points
3.3 Management of stormwater impacts
3.3.1 Detention and Infiltration
No on-site allowance will be made for the detention of stormwater on site. Overland runoff will
follow the existing drainage lines, with trapped low points detaining portions of runoff throughout
the site. Trapped low-points contain no formal outflow structure and drain through seepage
infiltration, recharging the upper, intergranular unconfined aquifer.
3.3.2 Treatment
To limit pollutant infiltration, side drains along the proposed road will be designed to have sediment
traps before discharging to low points.
3.3.3 Conveyance
For major storm events, the excess runoff will be routed along existing drainage lines and towards
trapped low points.
3.3.4 Continuity
In accordance with the City of Cape Town guidelines, the site must accommodate stormwater from
upstream catchments. Based on existing conditions, majority of the runoff from the catchment area
flows to numerous naturally occurring trapped low-points with no clear drainage path. The planned
access road will cut off only small isolated portions of overland flow. Crossing trapped low-points
has also been minimised. In these locations stormwater infrastructure will be constructed to permit
the balancing of runoff between upstream and downstream portions of the minor sub-catchments.
4 CONCLUSIONS & RECOMMENDATIONS
It may be concluded that:
1) Due to the rolling dune topography, naturally occurring trapped low-points and no clear
drainage path can be established.
2) The post-development runoff from the site will continue to flow along existing drainage lines
and towards existing trapped low-points for major events.
3) The horizontal alignment of the proposed access road is suitable, granted adequate
stormwater infrastructure be constructed to balance the runoff between upstream and
downstream portions of the minor sub-catchments.
5 REFERENCES
Depth Duration Frequency Rainfall Grid / auth. City of Cape Town. - Cape Town : City of Cape
Town, 2010.
Minimum Standards for Civil Engineering Services in Townships Version 1, July 2013, City of Cape
Town – Transport for Cape Town;
Guidelines for Human Settlement Planning and Design Volume 2, 2000, CSIR – Department of
Housing;
Management of Urban Stormwater Impacts Policy Version 1.1, May 2009, City of Cape Town:
Roads & Stormwater;
Stormwater Management Planning and Design Guidelines for New Developments, 2002, City of
Cape Town: Transport Roads & Stormwater;
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